mark add_len / set_len as deprecated

lib/cstruct.mli

@@ -320,9 +320,8 @@ val blit_to_bytes: t -> int -> bytes -> int -> int -> unit
     a valid substring of [dst]. *)
 
 val blit_to_string: t -> int -> bytes -> int -> int -> unit
-(** [blit_to_string] is a deprecated alias of {!blit_to_bytes}. 
-
-    @deprecated This is a deprecated alias of {!blit_to_bytes}. *)
+  [@@ocaml.deprecated "Use blit_to_bytes instead, blit_to_string will be removed in cstruct 5.0.0"]
+(** [blit_to_string] is a deprecated alias of {!blit_to_bytes}. *)
 
 val memset: t -> int -> unit
 (** [memset t x] sets all the bytes of [t] to [x land 0xff]. *)
@@ -333,11 +332,13 @@ val len: t -> int
     buffer, as the [sub] or [set_len] functions can construct a smaller view. *)
 
 val set_len : t -> int -> t
+  [@@ocaml.deprecated "This function will be removed in cstruct 5.0.0. If you need this function, discuss other ways in the issue tracker https://github.com/mirage/ocaml-cstruct."]
 (** [set_len t len] sets the length of the cstruct [t] to a new absolute
     value, and returns a fresh cstruct with these settings.
     @raise Invalid_argument if [len] exceeds the size of the buffer. *)
 
 val add_len : t -> int -> t
+  [@@ocaml.deprecated "This function will be removed in cstruct 5.0.0. If you need this function, discuss other ways in the issue tracker https://github.com/mirage/ocaml-cstruct."]
 (** [add_len t l] will add [l] bytes to the length of the buffer, and return
     a fresh cstruct with these settings.
     @raise Invalid_argument if [len] exceeds the size of the buffer. *)
@@ -371,7 +372,7 @@ val hexdump_to_buffer: Buffer.t -> t -> unit
 
 val hexdump_pp: Format.formatter -> t -> unit
 (** [hexdump_pp f c] pretty-prints a hexdump of [c] to [f]. *)
- 
+
 val debug: t -> string
 (** [debug t] will print out the internal details of a cstruct such
     as its base offset and the length, and raise an assertion failure